Platelet aggregation is a central event in the development of acute vascular thromboses. Fibrinogen binding to its platelet receptor (GPIIb-IIIa or integrin aIIbB3) is required for aggregation and thrombus formation. Although much is known about this important integrin, the specific molecules involved in its activation remain unknown. Marked gender differences exist both in the epidemiology of clinical thrombotic events and in platelet function. Thrombotic episodes in young women are more frequent during pregnancy and with oral contraceptive use, and acute coronary thrombosis carries a worse prognosis for women than men. We have demonstrated that platelets from women bind more fibrinogen to GPIIb-IIIa than platelets from men, that an increase in platelet fibrinogen binding correlates with the development of acute thrombosis, and that inhibition of platelet fibrinogen binding is more effective in women than men in decreasing acute coronary thrombosis. Furthermore, our preliminary data suggest that these gender differences are mediated by a sex hormone-induced alteration in megakaryocytes which has subsequent effects on circulating platelet function. The major hypothesis to be tested is that hormonally regulated megakaryocyte genes govern the activation of platelet fibrinogen receptors. The major objective of this project is to understand the molecular basis for gender differences in platelet fibrinogen binding. First, we will correlate in vivo platelet function with sex hormone levels. Second, we will study sex hormone regulation of megakaryocyte function using cultured human megakaryocytes. And third, we will identify megakaryocyte molecules differentially expressed between female and male megakaryocytes in response to sex hormones by: a) representational difference analysis cloning to isolate the relevant megakaryocyte cDNAs, and b) analyzing selected megakaryocyte signaling molecules. We hope that this work will enable us to reach our broader goals of furthering the understanding of platelet integrin function and understanding specifically the pathophysiology of acute vascular thromboses in women.